Method of producing a ferrite body and product obtained thereby



Jan. 29, 1963 M. B. vlLENsKY 3,075,920 METHOD oF PRDDUCING A FERRITEBODY AND PRODUCT OBTAINED THEREBY Filed May 26, 1961 INyENToR. W ,//L DHM n Byfaz@ '27. 47%@ :E J: IE- L7 7'0 VACUUM PUMP Arroz/VE Y llnitedtates .datent @bien lz@ Patented dan. 29, ld

3,975,926 METHD @iF PRQDUCENG A. BDY AND PROEUCT GBTAlNED 'flllllllllllMichel E. Vilensky, San Francisco, Calif., assigner to Amper;Corporation, Redwood City, Calif., a corporation of California Filed May26, i963., Ser. No. 312,965 lll Claims. (Qi. 252-62.5)

The present invention relates to ferrite materials, and particularly tomethods of producing compact ferrite bodies, and the products obtainedby such methods.

Many ferrite materials are now well known, and bodies constructed ofsuch materials have been widely used in the construction of varioustypes of electronic equipment, such as magnetic recording heads,microwave devices, and antennas. Ferrite materials usually consist of amixture of the oxides of particular bivalent metals (e.g., nickel andzinc) with ferrie oxide, Fe203, these being formed into a mass andheated to sintering temperature to produce the ferrite body.

The known ferrite bodies have usually been subject to one or more of anumber of disadvantages. Being `made by a sintering process, theirphysical strength and ductility have usually been low, so that suchbodies are diiloult to fabricate. For the same reasons they are readily.damaged and have relatively short operating lifetimes. Physicalcharacteristics have also suffered because these ferrite bodies haveusually been of comparatively low density due to a high degree ofporosity. EX- cess porosity also introduces a tendency to permit theundesirable passage of gases and moisture, causing breakage of a part,or the formation of an occluded structure. These factors invariablyintroduce poor magnetic qualities, such as low and non-uniform magneticpermeability.

A number of diiferent techniques have been employed in the prior art forreducing the above physical and magnetic disadvantages found in ferritebodies. lmpregnation has been tried, but has not overcome the tendencyto brittleness and cracking. Additionally, foreign materials have beenintroduced, markedly decreasing the permeability and other magneticcharacteristics of the fabricated part.

It is therefore an object of the present invention to provide improvedmethods of producing ferrite bodies having superior combinations ofphysical and magnetic properties.

Another object of the invention is to provide a method of producing animpregnated ferrite body to obtain greater strength and ductility thanhas heretofore been available.

Another object of the present invention is to provide a method offorming a ferrite body to provide a greater density and lower porositythan has been heretofore obtained.

Yet ano-ther object of the invention is to provide improved ferritebodies having high density, good physical characteristics and excellentmagnetic properties.

Dense and strong ferrite bodies are provided in accordance with theinvention by preparing a compact, in the desired form, of the materialsfor the needed ferrite composition. The compact is then at leastpartially fired for sintering and to convert the composition to thespinel crystal structure. After the initial firing, the body derived isimmersed in room temperature in a solution of chosen salts, and vacuumimpregnated to release contained gases and to fill the pores of the bodywith the salt residue. After drying, the body is subjected to a iinaltiring sequence in which the salts are converted to the correspondingoxides. ln accordance with the invention, the chosen salts are of suchcomposition and in such proportion that on conversion to the oxide formthey have an identical chemical composition to the ferrite body itself.

A. better understanding of the invention may be had from the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FlGUlE 1 is a flow sheet, representing diagrammatically a process inaccordance with the invention;

FlGURE 2 is a diagrammatic view of impregnation apparatus employed inaccordance with the invention;

FIGURE 3 is a representation of a photomicrograph of a portion of aferrite body which has not been impregnated; and

FlGURE 4 is a representation of a photomicrograph similar to FIGURE 3,in which the ferrite bo-dy has been impregnated in accordance with theinvention.

Referring to the drawings, and particularly to FIGURE l, there is showna process for the production of a high density ferrite body. As a iirststep in the process, a batch of ferrite material of any known type maybe prepared by conventional means. The ferrite employed may, forexample, be of the type containing nickel (or a combination of nickeland copper) and zinc oxides in combination with Fe203; zinc andmanganese oxides in combination with Fe203; or nickel and magnesiumoxides in combination with Fe203; or other known oxide combinations.Such oxides are preferably utilized in the form of very fine grains.Homogeneity of the selected oxides may be produced through the use of astainless steel ball mill, the milling being carried out in stainlesssteel containers loaded with required numbers yof steel balls with theaddition of water. In addition, the milling promotes a solid statereaction among these oxides. After milling, the material is screenedusing 40 mesh size screen, dried at room temperature, and thenpulverized with a hand mortar. The material is then preliminarily redfor several hours at a temperature at which spinel is formed (e.g.,S60-1100" C), and cooled to room temperature. Relatively little spinelstructure is present at this point. Further ball milling may berequired, and the material is then finally screened 4to pass particlesof approximately 40 mesh size or less. Finer particles usually provide asuperior structure.

A quantity of the oxides processed in this manner is then formed into abody of any ldesired shape. The present procedure, given by way ofexample, is for the fabrication of a recording head component. Theinitial for-ming may be accomplished by placing the particles, togetherwith a binder (e.g., 1% by weight of a resin alone or a resin landlubricant combined) under pressure in a properly configured die. A steeldie and a hydraulic pressure of -8 tons per square inch have been foundsuitable. The compact thus obtained is in the desired shape and may thenbe dried by leaving the body at room temperature for in excess of 24hours. After this interval the binder may be decomposed and eliminatedby heating up -to 400 C., a slow firing in an electric 4furnace beingemployed for best results.

The formed body is then fired in order to effect either partial orsubstantially comple-te conversion of the metallic oxides to the spinelstructure and 'to sinter the compacted particles. A schedule of severaltiring steps is followed, the times and temperatures employed `dependingon the oxide composition of the body and the degree of conversiondesired, with a partial conversion being preferred. For example, in thecase of a nickelzinc ferrite body (l5 -mol percent NiO, 35 mol percentZnO and 50 mol percent FeZOB), it has been found advantageous to iirstheat the body slowly to about 400 C., at which temperature the body isheld for about 2 hours. During this time, migration of atoms from onelattice form to another occurs in the material. rf'he body is thenheated in the next step at about 600 C.

amasar) for about 4 hours. At this temperature, some spinel starts toform. The temperature then is raised gradually to about 900 C., at whichtemperature more complete formation of the characteristic spinelstructure takes place. This temperature is maintained for at least 4hours. A longer interval, of from 6-8 hours, is not harmful. Finally,lthe temperature may be increased to about ll C., which temperature isheld for about l-2 hours, but not less than 1 hour, as a preparatorymeasure for the impregnation procedure to be discussed below. Thepreliminary firing and sintering imparts physical strength to the body,which would otherwise be easily deformed or disintegrated duringhandling.

The partially tired, formed body which results from sintering themetallic oxide is cooled to room temperature by any convenient method,such as, for example, by cooling in the furnace overnight. A rate ofcooling of at least 10 C. per minute, is required for best results. Thebody is then subjected to the vacuum impregnation operation (see alsoFEGURE 2). As may be seen, the ferrite body l0 is placed in a containersuch as a bell jar 12 coupled by a hose 13 to a vacuum pump (not shown).An impregnating solution 16 is contained in a vessel, such as a beakerl?, set in the bell jar 12. The solution lo contains salts (e.g., thesulphate salts) of the same metals as are present in the ferrite body10, the salts being dissolved in H2O. Salts of the highest purityobtainable, i.e., electronic grade should be used. The salts aredissolved in a proportion accurately calculated to form, upon tiring,oxides corresponding in percentage composition to the oxides making upthe ferrite body lo itself. For the vacuum impregnation operation, theremay be employed the Kinney PW Packaged High Vacuum Pumping Unit PW-6G0(trade name), manufactured by Kinney M.F.C. Division of the New York AirBrake Company.

When the vacuum pump is set into operation, a high vacuum (c g., up to30 in. of Hg) is gradually applied to the system, so that any gas suchas air occluded in the pores of the ferrite body 10 is evacuated. At thesame time, some of the salt solution 16 in which the ferrite body l0 isimmersed enters the evacuated pores. The air or other gas in the body l0escapes as bubbles in solution, and the end point of the impregnationprocedure is observed when no more bubbles appear. The pressure is thenheld for an additional time (eg. 1/2 hour) and then gradually returnedto normal.

Following the impregnation procedure, the ferrite body l0 is removedfrom the bell jar l2 and dried by any conventional method, preferably atroom temperature in air. The gases occluded in the pores of the body 10have at this point been evacuated and the pores at least partly filledand sealed with the metallic salts, and reduced considerably from theiroriginal size. With salts of adequate purity no impurities areintroduced into the structure.

The resulting impregnated ferrite body lill is then again placed in afurnace (not shown) for a linal tiring. If the ferrite body l0 has onlybeen partially converted to the spinel structure in the original firing,the final ring is designed in part to complete the conversion and thesintering process. Such final firing, moreover, also thermallydecomposes the impregnated metallic salts into their correspondingcomplex oxides. if the ferrite body has been fully converted in theoriginal tiring, the final firing will, of course, serve only to convertthe salts. Here, again, the temperature and time of tiring will dependupon the metallic salts to be converted, and the temperature cycle whichthe ferrite body has previously undergone in the original bring. Theoriginal firing schedule, already described, is preferably employedagain here. After the final step (at whichllGO" C. is reached), thetemperature is increased to final tiring temperatures of about l300 C.,for about 4-5 hours; and of about l3501400 C. for about l hour.

Subsequent to the final firing, the ferrite body l) is cooled to roomtemperature, preferably by one of three methods, depending upon thenature of the ferrite: furnace cooling, air quenching or nitrogencooling. In the case of nickel-zinc ferrites, preliminary cooling in thefurnace to 600 C. is preferred, following by removal from the furnace(or complete furnace cooling). Air quenching has the advantage ofarresting the size of crystal growth and providing a stronger ferrite.Nitrogen cooling, from 11d()o C., down to room temperature is preferredfor manganese-type ferrites, in order to avoid non-magnetic structures.

One or more additional vacuum impregnations of the ferrite body 16 mightin some instances be desirable, prior to tiring, in order to providesufficiently great density of the final product. It is found howeverthat, with proper operating conditions a single procedure is suflicient.

Ferrite bodies which have been impregnated by salts which areA convertedto oxides in accordance with the present invention have been found tohave substantially greater density than non-impregnated ferrite bodiesof the same metallic oxide composition, as well as a much lesser degreeof porosity. The oxides combine with the body of the ferrite to form ahomogeneous structure of identical chemical composition. The pores aresealed and the structure has markedly improved physical characteristicsas Well as improved magnetic uniformity. Referring to FIG- URE 3 aportion l of a nickel-zinc type ferrite body is shown at a magnificationof 40G, having been prepared in conventional manner by tiring a ferritebody composed of 15 mol percent NiO; 35 mol percent ZnO; and 50 molpercent H2203. The ferrite body portion 19 is to be compared with theferrite body portion 20 shown in FIGURE 4, which has been prepared frommetallic oxides of the same composition, but in accordance with theinvention. The difference in porosity and uniformity as between theportions 19 and 20 is obvious, the small enclosed areas de fining theporous zones of the material. The portion 19 might, for example, have adensity of about 4.8 as compared to a density of about 5.1 for theportion 20.

Example I A batch of material containing metallic oxides was prepared inthe following proportions for a nickel-zinc ferrite:

After initial grinding to the desired tineness and inclusion of abinder, a portion of the above batch was pressed into the shape oftwelve bars, each about 1%" x 1/s" x /g, a hydraulic press operated atabout 5 tons/in.2 being employed. The bars were then slowly heated in anelectric furnace to a temperature of about 400 C., and maintained atthis temeperature for about 2 hours. The bars were then heated to about600 C., for about 4 hours. Thereafter, the bars were slowly heated to900 C., and held at this temperature for about 16 hours. The temperaturewas then increased to and held at 1100" C. for about 2 hours, afterwhich the bars were furnace cooled for a period of about l5 hours, toroom temperature.

The partially converted spinel structure ferrite bars thus prepared werethen placed in a bell jar, connected by tubing to a beaker containingimpregnating solution having the following composition of saltsdissolved in ml. of distilled water:

Grams NiSO4-6H2O 6.64 ZnSOyI-lzO 17.54 FeSO4-7H2O 23.20

The bell jar was then subjected to a vacuum of about 30 in. of Hg, for aperiod of about 11/2 hours, this being about 1/2 hour longer thanescaping air bubbles were visible. After slow release of the vacuum, theferrite bars were then removed from the solution and allowed to dry atroom temperature.

The impregnated bars were then fired, employing the same firing scheduleset forth above. Following the final state (at 1l00 C.), they werefurther heated to about 1300 C. for about 4 hours; and then to about1400 C. for about 1 hour. The nickel-zinc ferrite bars were thereaftercooled to about 600 C. in the furnace; and finally air quenched. Thebars, upon examination, were found to have a density of 5.12 grams/cm3.Prior to vacuum impregnation, the density was determined to be 4.82grams/cm3.

Example 2 Zinc-manganese ferrite bars were also prepared from a batchhaving the following composition:

Molecular ,Y percent MnO 17 ZnO 33 F5203 Another type of nickel-zincferrite bar was prepared from a batch having the following composition:

Mol percent NiO 17.50 ZnO 33.20 FeZOS 49.30

After initial tiring and vacuum impregnation in a sequence similar tothat of Example 1, a marked change in porosity was effected (see FlGURE4). The density of the bar was increased from 4.88 grams/cm.3 to 5.16grams/cm?. Example 4 In another example of a nickel-zinc ferrite, thefollowing successive steps were used in ring prior to impregnation:

Hours 400 C. 2 900 C. 4 l000ll00 C. 4-

Thereafter the ferrite was vacuum impregnated in sulphate salts ofnickel, zinc and iron, selected in accurate proportions to form oxidescorreseponding to the ferrite composition. Following drying, thefollowing selected tiring schedule was used:

Hours 400 C. 2 900 C. 4 1100 C 12 13G0 C. 4 1360 C. 1

The ferrite was then cooled back to room temperature according to thefollowing schedule:

From 1360 C. to l100 C.2 hours; From l100 C. to 600 C.4 hours; At 600C.6 hours; and then air quenched.

A similar increase in density of about 0.405 gram/cm.3 was observed.

lt will be appreciated that various alternatives in addition to thosegiven Will present themselves to those skilled in the art. Also, theferrite material formed in accordance with this invention may be usefulfor the manufacture of various magnetic structures other than recordingheads such as magnetic cores that are used in stacked arrays.Accordingly, the invention should be considered to include allmodifications and variations falling Within the scope of the appendedclaims.

What is claimed is:

1. The method of producing an impregnated ferrite body which comprisesthe steps of subjecting a sintered ferrite body to vacuum impregnationby a salt solution, the proportions of which are selected to form oxidesupon tiring having the same chemical composition as the body, thereafterdrying the impregnated ferrite body, and then firing the impregnatedferrite body to thermally decompose the impregnation salts and formoxides corresponding in composition to the body.

2. The method of producing a dense and strong ferrite body whichincludes the steps of subjecting a partially sintered ferrite bodyhaving metallic oxides to vacuum im` pregnation by a salt solution whichupon firing forms oxides having the same chemical composition as thebody, thereafter drying the impregnated ferrite body, and then firingthe impregnated ferrite body to thermally decompose the impregnationsalts and form the corresponding oxides, the firing further completingthe sintering of the metallic oxides originally in the body andsintering the decomposition oxides.

3. The method of producing a ferrite body which includes the steps ofsubjecting an at-least partially sintered ferrite metallic oxide body tovacuum impregnation by a salt solution at least twice which upon tiringforms oxides having the same chemical composition as the ferrite-typebody, drying the impregnated ferrite body following each impregnationstep, and firing the impregnated ferrite body to thermally decompose theimpregnation Salts and form the corresponding oxides after each dryingstep, the ring further sintering the decomposition oxides.

4. The method of producing a ferrite body which includes the steps offorming a quantity of sintered ferrite having metallic oxides into abody of predetermined shape, tiring the body at temperatures and fortimes suicient to partially sinter the metallic oxides, cooling theferrite body to room temperature, subjecting the ferrite body to vacuumimpregnation by a salt solution of metallic salts corresponding to themetallic oxides and in proportions which upon firing form oxides havingthe same chemical composition as the body, drying the impregnatedferrite body, and firing the impregnated ferrite body at temperaturesand for times which thermally decompose the impregnation salts and formthe corresponding oxides, the firing further sintering the decompositionoxides and completing the sintering of the originally present metallicoxides.

5. rlhe method of producing a relatively dense ferrite body whichincludes the steps of forming a sintered ferrite having metallic oxidesinto a body of selected shape, tiring the body at temperatures and fortimes to partially convert the oxides to the spinel crystal structureand to sinter the metallic oxides, subjecting the ferrite body to vacuumimpregnation in a salt solution which upon firing forms oxides havingthe same chemical composition as the body, and finally tiring theimpregnated ferrite body to thermally decompose the impregnation saltsinto the corresponding oxides and to fully convert the original metallicoxides into the spinel crystal structure.

6. The method of producing a dense ferrite body which includes the stepsof subjecting a sintered ferrite body having metallic oxides to vacuumimpregnation by a salt solution selected to form oxides upon firinghaving the same chemical composition as the body, ring the impregnatedferrite body to thermally decompose the impregnation salts and form thecorresponding oxides, the tiring further sintering the decompositionoxides, and finally cooling the body to room temperature.

7. The method of producing a ferrite body which includes the steps ofsubjecting a sintered ferrite body having metallic oxides to vacuumimpregnation by a solution of metai sulphates which upon tiring formmetallic oxides having the same chemical composition as the body, dryingthe impregnated ferrite body, and firing the impregnated ferrite body tothermally decompose the Iimpregnation salts and form the correspondingoxides.

8. The method of producing a ferrite body, comprising the steps ofplacing a sintered ferrite body having metallic oxides in a container,preparing a salt solution of metallic salts corresponding to themetallic oxides of the body, evacuating air from the container,consequently importing but suction a portion of the salt solution intothe container for impregnation of the body, drying7 the impregnatedferrite body, and firing the impregnated ferrite body to thermallydecompose the salts into the corresponding oxidesand fully convert themetallic oxide body into the spinel crystal structure.

9. A ferrite body, consisting essentially of a ferrite body ofsubstantially completely sintered original metallic oxide o material,the pores of the body being at least partially sealed after drying bysintered thermally decomposed oxides of metallic salts that haveimpregnated such pores by vacuum means, the oxides of decompositioncorreg spending in chemical composition to the original metallic oxidesof the ferrite body.

l0. A dense and strong ferrite body, consisting essentially of a ferritebody of substantially completely sintered original metallic oxidematerial, the pores of the body including sintered thermal decompositionoxides of metal sulphates that have impregnated such pores by vacuummeans, the decomposition oxides corresponding in chemical composition tothe original metallic oxides of the ferrite body.

References Cited in the file of this patent FOREIGN PATENTS 739,069Great Britain Oct. 26, 1955

1. THE METHOD OF PRODUCING AN IN PREGRETED FERRITE BODY WHICH COMPRISESTHE STEPS OF SUBJECTING A SINTERED FERRITE BODY TO VACUUM IMPREGNATINGBY A SALT SOLUTION, THE PROPORTIONS OF WHICH ARE SELECTED TO FORM OXIDESUPON FIRING HAVING THE SAME CHEMICAL COMPOSITION AS THE BODY THEREAFTERDRYING THE IMPREGNATED FERRITEBODY, AND THEN FIRING THE IMPREGNATEDFERRITE BODY TO THERMALLY DECOMPOSE THE IMPREGNATION SALTS AND FORMOXIDES CORRESPONDING IN COMPOSITION TO THE BODY.